The invention relates to a guide for a spherically curved window pane which can be lowered in a door shaft through a special design of the guide rails of a double-strand cable window lift which can be made by slight modifications to the vehicle coachwork without incurring substantially increased manufacturing costs. More particularly, the invention allows cable window lifts with substantially parallel construction suitable for lifting and lowering spherically curved window panes.
A spherically curved window is curved in three dimensions and typically has different radii of curvature at the front and at the rear of the window which defines a smaller radii of curvature and a larger radii of curvature, respectively. To raise and lower such a window in a motor vehicle door requires different design solutions than those for flat window panes or window panes curved in only two dimensions.
German Patent No. 40 08 229 Al discloses a device for lifting and lowering a vehicle window pane with two closed cable loops which rotate in opposite directions to each other. Two cable drums are mounted on separate parallel axles and are in a friction or keyed engagement with each other. One of the two cable drums is driven by a manual or electrical unit. Each of the two cable loops is guided along a substantially vertical guide rail over cable guides provided at their ends.
According to a variation of this device, a combination of cable drums with different diameters is provided whereby a corresponding translation ratio is produced between the two cable loops. Thus it is possible to adapt the window lift to the special passage conditions inherent to spherically curved window panes. The guide rail with the slower displacement speed and small displacement path of the entrainment member, or slider, is mounted on the side of the smaller pane radius and the guide rail with the faster displacement speed and larger displacement path of the entrainment member is mounted on the side of the larger pane radius.
However, there is a drawback in the comparatively high technical expense which is necessary in order to displace severely spherically curved window panes with such a device as the double construction of the cable loop and cable drum leads to significantly higher costs.
German Patent No. 37 18 840 Cl discloses displacement of spherically curved window panes by means of a conventional double-strand cable window lift which has two guide rails with conforming curvature and whose entrainment members (sliders) cover equal length paths during operation of the window lift causes a tilting movement during lowering of the window pane which leads to the window pane bearing with force against the door shaft at least one point. This leads to tensions in the window lift system and in the door body. Furthermore there is the disadvantage that the increased system friction requires an increased drive moment and thus the use of more powerful and more expensive motors.
It is desirable to develop a vehicle door with a double-strand cable window lift so that a sufficiently precise parallel lowering/raising passage of the lower edge of the window pane is accomplished even in the case of severely spherically curved window panes. This further development is to be achieved without expensive additional measures and additional parts.